The objective of the research contained in this application is to elucidate how the structural features of carotenoids determine their photochemical properties in vivo. Carotenoids are essential for the survival of photosynthetic organisms. The systems to be studied in this investigation are several different photosynthetic bacterial reaction center and antenna complexes that have been isolated, pigment-reconstituted, pigment-exchanged, crystallized or genetically altered. X-ray crystallography and NMR analyses will be used as direct structural probes. Optical and EPR spectroscopy will allow the correlation of structure with spectroscopic observables and function. These data will provide a basis from which to understand more complex systems; e.g. higher plant pigment-protein complexes whose properties are more complex than the bacterial systems. The specific aims of this proposal are: (1) To determine the structures of carotenoids in photosynthetic pigment-protein complexes; (2) To determine the relationships between the carotenoid structures and their spectroscopic properties; (3) To determine the relationships between the carotenoid structures and their mechanisms of carotenoid-to-bacteriochlorophyll singlet and bacteriochlorophyll-to-carotenoid triplet energy transfer. Several preliminary studies have already been carried out by us in support of the methods which we propose to employ.